Continuing innovations in the field of fiber optic technology have contributed to the increasing use of fiber optics in communication networks. The flexibility and reliability of optical communication networks are greatly increased by the availability of nonreciprocal optical devices such as optical circulators and optical isolators. Optical circulators enable a bi-directional optical fiber to be coupled to both an input optical fiber and an output optical fiber. Optical isolators provide forward propagation of optical signals through an optical fiber, while inhibiting unwanted back reflection and scattering.
Optical isolators are particularly useful when used in optical communication networks with devices that are sensitive to spurious reflections. As an example, some lasers tend to be unstable if the laser output is reflected back to the laser. As another example, reflected optical signals can cause an optical amplifier to oscillate, which may adversely affect the operation of the amplifier.
A common optical isolator includes a combination of walk-off crystals, wave plates and Faraday rotators. The walk-off crystals are typically used to selectively displace the orthogonal polarization components of optical signals to separate and/or combine the optical signals. The wave plates are used to provide reciprocal rotation to the polarization components of the optical signals. Reciprocal rotation means that the rotational direction for forward propagating polarization components is counter to the rotational direction for rearward propagating polarization components when viewed from a fixed reference point. The Faraday rotators provide nonreciprocal rotation to the polarization components of the optical signals. Nonreciprocal rotation means that the rotational direction for forward propagating polarization components is the same as the rotational direction for rearward propagating polarization components when viewed from a fixed reference point. The rotations caused by the wave plates and the Faraday rotators are such that only forward propagating optical signals from an input optical fiber are transmitted to an output optical fiber through the optical isolator. Thus, rearward propagating optical signals from the output optical fiber are not transmitted to the input optical fiber through the optical isolator. Consequently, reflected optical signals are not transmitted back to a light source or an optical amplifier.
Although optical isolators are useful for the operation of optical communication networks, the same optical isolators impede network measurements due to their nonreciprocal function. As an example, Optical Time Domain Reflectometer (OTDR) is a powerful network tool to characterize an optical fiber. OTDR is used to estimate optical fiber length and overall attenuation, including splice and mated-connected losses, and to locate a breaking point position of the optical fiber by measuring the reflection and scattering of injected optical pulses. However, optical isolators in an optical communication network significantly reduce the amount of light that is reflected and scattered. Thus, the optical isolators tend to dramatically suppress the measurability of the optical communication network by an OTDR. A common solution to this problem is to use jumper fiber cables to bypass the optical isolators in the optical communication network. However, the use of jumper fiber cables is an intrusive method that requires extensive labor and long recovery time for the optical communication network.
In view of this concern, there is a need for an optical isolator that does not suppress the measurability of optical communication networks.